Since liquid crystal displays have features of being thin, low in power consumption and others, the use thereof has been expanding in various articles from large-sized displays to portable information terminals and the development thereof has been actively made. Conventionally, for liquid crystal displays, a TN system, an STN multiplex driving system, an active matrix driving system in which thin film transistors (TFTs) are used in TN, and others have been developed and made practicable. However, since nematic liquid crystal is used therein, the response speed of the liquid crystal material is as small as several milliseconds to several tens of milliseconds and it cannot be said that these sufficiently cope with display of moving images.
Ferroelectric liquid crystal (FLC) exhibits a very fast response in order of microseconds, and thus FLC is a liquid crystal suitable for high-speed devices. About ferroelectric liquid crystal, there is well known a bistable liquid crystal which has two stable states when no voltage is applied thereto and is suggested by Clark and Lagerwall (FIG. 14 upper graph). However, the liquid crystal has a problem that the liquid crystal has memory properties but gray scale display cannot be attained since the switching thereof is limited to two states, namely, bright and dark states.
In recent years, attention has been paid to ferroelectric liquid crystal in which the liquid crystal layer thereof is stable in a single state (hereinafter referred to as “monostable”) when no voltage is applied thereto as a liquid crystal making it possible to attain gray scale display by the matter that the director (the inclination of the molecule axis) of the liquid crystal is continuously changed by a change in applied voltage so as to analogue-modulate the light transmission thereof (Non-Patent Document 1, FIG. 14 lower graphs). As the liquid crystal showing the mono-stability, ferroelectric liquid crystals having the phase change of cholesteric phase (Ch)-chiral smectic C phase (SmC*) without the transition to the smectic A (SmA) phase in the temperature lowering process are generally used (FIG. 13 upper part).
On the other hand, as the ferroelectric liquid crystal, there is a material having the phase change of cholesteric phase (Ch)-smectic A phase (SmA)-chiral smectic C phase (SmC*) so as to show the SmC* phase via the SmA phase in the temperature lowering process (FIG. 13 lower part). Among the ferroelectric liquid crystal material reported so far, most of them are those having the latter phase sequence of passing through the SmA phase compared with the former material which does not pass the SmA phase. It is known that the latter ferroelectric liquid crystal having the phase sequence of passing through SmA phase in general has two stable states with respect to one layer normal line so as to show the bi-stability.
In recent years, color liquid crystal displays have been actively developed. The method for realizing color display is generally classified into a color filter system and a field sequential color system. The color filter system is a system of using a white light source as a back light and attaching a micro color filter in R, G or B color to each pixel, thereby realizing color display. On the other hand, the field sequential color system is a system of switching a back light into R, G, B, R, G, B . . . with time, and opening and shutting a black and white shutter of a ferroelectric liquid crystal in synchronization therewith to mix the colors with time by afterimage effect on the retina, thereby realizing color display. This field sequential color system makes it possible to attain color display in each pixel, and does not require any color filter low in transmission. As a result, this system is useful since the system is capable of attaining bright and highly precise color display and realizing low power consumption and low costs.
The field sequential color system is a system in which each pixel is subjected to time sharing; it is therefore necessary for the liquid crystal as the black and white shutter to have high speed response properties in order to give good moving image display properties. If ferroelectric liquid crystal is used, this problem can be solved. The ferroelectric liquid crystal used at this time is, in particular, desirably a liquid crystal exhibiting mono-stability in order to make gray scale display based on analogue modulation possible and realize highly precise color display, as described above. As the ferroelectric liquid crystal exhibiting mono-stability, there are a ferroelectric liquid crystal which responds to both of plus-polarized and minus-polarized voltages (FIG. 14 right lower graph) and a ferroelectric liquid crystal which responds only to either of plus- or minus-polarized voltage (FIG. 14 left lower graph). When a ferroelectric liquid crystal is driven by using a thin film transistor (TFT) element, it is particularly preferable to use the ferroelectric liquid crystal which responds only to either of plus- or minus-polarized voltage since effect of inversion current generated by spontaneous polarization is less.
Herein, FIG. 15 shows an example of a driving sequence of a liquid crystal display based on a field sequential color system using a TFT element. In FIG. 15, it is supposed that the voltage applied to the liquid crystal display is set into the range of 0 to ±V (V), data-writing scanning is attained through a plus-polarized voltage, and data-erasing scanning is attained through a minus-polarized voltage. It is also supposed that a ferroelectric liquid crystal used exhibits mono-stability and responds to only either of plus- or minus-polarized voltage.
As illustrated in FIGS. 12A and 12B, the response of the ferroelectric liquid crystal which exhibits mono-stability and responds to only either of plus or minus-polarized voltage is classified to a case that the liquid crystal gives a response to a plus-polarized voltage to turn into a bright state (FIG. 12A), and a case that the liquid crystal gives a response to a minus-polarized voltage to turn into a bright state (FIG. 12B). As illustrated in FIG. 15, therefore, in the case of using the ferroelectric liquid crystal exhibiting the response illustrated in FIG. 12A (liquid crystal response 1), the liquid crystal turns into a bright state when a plus-polarized voltage is applied thereto. In the case of using the ferroelectric liquid crystal exhibiting the response illustrated in FIG. 12B (liquid crystal response 2), the liquid crystal turns into a bright state when a minus-polarized voltage is applied thereto.
The ferroelectric liquid crystal using a TFT element is driven by applying a certain voltage to a common electrode of a common electrode substrate facing to a TFT substrate and by applying a voltage to a pixel electrode of each pixel in the TFT substrate. Here, the application of a plus-polarized denotes to a case when the voltage applied to the pixel electrode is relatively higher than the voltage applied to the common electrode, and the application of a minus-polarized voltage denotes to a case when the voltage applied to the pixel electrode is relatively lower than the voltage applied to the common electrode.
When the ferroelectric liquid crystal which responds only to a plus-polarized voltage is used as a ferroelectric liquid crystal, an application of plus-polarized voltage (writing) and an application of minus-polarized voltage (erasing) are conducted in turn to strike a balance in charges. When a TFT element is used to drive a ferroelectric liquid crystal, voltages cannot be written into all the pixels simultaneously and scanning is performed in after another line. Thus, when scanning is performed from a first line to an Lth line, a time gap is generated between the writing scanning. Time gap is also generated in erasing scanning. In an example shown in FIG. 15, erasing scanning starts from a first line after all the writing scanning are completed.
Further, in a field sequential color system, writing scanning and erasing scanning are performed in synchronization with the flashing of the used back light. In FIG. 15, “+(R)” represents a matter that writing scanning (the application of a plus-polarized voltage) is performed in synchronization with a red (R) back light, and “−(R)” represents a matter that erasing scanning (the application of a minus-polarized voltage) is performed in synchronization with the red (R) back light. Similarly, “+(G)”, “−(G)”, “+(B)”, and “−(B)” represent matters that the scanning operations are performed in synchronization with a green (G) back light and a blue (B) back light, respectively.
As described above, in a field sequential color system, writing scanning and erasing scanning are performed in synchronization with the switching of the used back light into R, G, B, . . . with time, thereby causing the ferroelectric liquid crystal to respond. Accordingly, when scanning is performed in synchronization with the back light R, the liquid crystal turns into a bright state during the lighting of the back light R in each of the writing scanning (+(R)) on the first line and the writing scanning (+(R)) on the Lth line in the case of using the ferroelectric liquid crystal exhibiting the liquid crystal response 1. On the other hand, in the case of using the ferroelectric liquid crystal exhibiting the liquid crystal response 2, a time gap is generated in writing scanning (+(R)) and erasing scanning (−(R)) between the first line and the Lth line. Accordingly, by the erasing scanning (−(R)) on the Lth line in synchronism with the back light R, the liquid crystal turns unfavorably into a bright state when the back light G lights (broad line frames in FIG. 15). When scanning is performed in synchronization with the back light G, erasing scanning (−(G)) on the Lth line in synchronization with the back light G causes the following: the liquid crystal turns unfavorably into a bright state when the back light B lights (broad line frames in FIG. 15).
In FIG. 15, “Bright (R)” represents a matter that the liquid crystal turns into a bright state by scanning in synchronization with the back light R (red), and “Dark” represents a matter that the liquid crystal turns into a dark state by scanning in synchronization with each of the back light R (red), G (green) and B (blue). In the same manner, “Bright (G)” and “Bright (B)” represent matters that the liquid crystal turns into a bright state by scanning in synchronization with the back lights G (green) and B (blue), respectively.
In common liquid crystal display devices, it is generally decided that writing scanning and erasing scanning are each performed by either one of plus-polarized and minus-polarized voltages and cannot be changed easily; therefore, in order to avoid the above-mentioned problems, it is necessary to adjust the polarity of the voltage to which the ferroelectric liquid crystal exhibiting mono-stability responds to the polarity of the voltage applied. This response of the ferroelectric liquid crystal is decided by the direction of spontaneous polarization of the ferroelectric liquid crystal. Therefore, the polarity of the voltage to which the ferroelectric liquid crystal responds can be controlled if the direction of the spontaneous polarization can be controlled.
Ferroelectric liquid crystal has a higher molecule order than nematic liquid crystal, so as not to be aligned with ease. In particular, in ferroelectric liquid crystal which does not pass SmA phase, two domains different in the layer normal line direction (referred to as the “double domains” hereinafter) are generated (the upper part in FIG. 13). In such double domains, white-black reversed display is generated when the liquid crystal is driven. This becomes a serious problem. For this reason, various alignment treatments are being investigated.
As a method for overcoming the double domains, known is, for example, the electric field induced technique of heating a liquid crystal cell to a temperature not lower than the cholesteric phase, and cooling the liquid crystal cell gradually while applying a DC voltage thereto (see Non-Patent Document 2). When this electric field induced technique is used, the direction of the spontaneous polarization can be controlled in accordance with the direction of the applied electric field. According to this method, however, if the temperature is raised again to the phase transition point or higher, alignment disorder is generated. Moreover, manufacturing process becomes complicated and there is caused a problem such as that alignment disorder is generated in the region on which the electric field does not act between pixel electrodes.
As a method to mono-stabilize a ferroelectric liquid crystal, a method of using photo alignment layer as the upper and lower alignment layers, wherein a material having different composition is used for each of the photo alignment layers, is proposed (Patent Documents 1-3). The reasons of obtaining a good alignment state by employing a material having different composition for the each of the upper and lower photo alignment layers are not clear, but it is thought to be a difference in interaction between the respective photo alignment layer and the ferroelectric liquid crystal. A direction of spontaneous polarization of liquid crystal display obtained by such a method, however, cannot be known until the liquid crystal display is actually driven.
As another method to mono-stabilize a ferroelectric liquid crystal, a method, wherein a reactive liquid crystal is coated to either of the upper and lower alignment layers to align and fix the liquid crystal, and a fixed liquid crystal layer (reacted liquid crystal layer) is formed to make the fixed liquid crystal layer act as an alignment layer, is proposed (Patent Document 4). In this method, since the structure of a reactive liquid crystal is relatively similar to that of a ferroelectric liquid crystal, an interaction between the alignment layer and the ferroelectric liquid crystal becomes strong. Thus, an alignment of liquid crystal can be controlled more effectively than a case of using only photo alignment layers, and the ferroelectric liquid crystal can be aligned without causing alignment defects such as double domains by forming a fixed liquid crystal layer to either of the upper and lower alignment layers. However, a direction of spontaneous polarization of liquid crystal display obtained by such a method cannot also be known until it is actually driven.    Patent Document 1: Japanese Patent Application Publication 2005-208353    Patent Document 2: Japanese Patent Application Publication 2005-234549    Patent Document 3: Japanese Patent Application Publication 2005-234550    Patent Document 4: Japanese Patent Application Publication 2005-258428    Non-Patent Document 1: NONAKA, T., LI, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599.    Non-Patent Document 2: PATEL, J., and GOODBY, J. W., 1986, J. Appl. Phys., 59, 2355.